Ok, so you don't already have a wireless router? If not, then sure an Airport Express (or Airport Extreme) will help you transport your data, but then if you're trying to play the content in another room, you'll see need something for it to play back on.
The easiest solution for sharing mainly your music to your home theater (given most of your songs are in iTunes) is still the Apple TV.
If you're looking to run your PC to your HT directly, you might want to start looking into new graphics and/or sound cards which can deliver the signals you need via hdmi/dvi and optical audio jacks. Also, depending on the lengths, you may need higher grade cables to prevent a signal drop off. (someone else here probably knows more about where hdmi lengths might start to give you problems or other solutions.)
I also use Tversity to stream music/video from my PC to my PS3, but I don't think that works with AAC files from iTunes.
